1. Field of the Invention
The present invention relates to an ink tank and a recording apparatus.
2. Description of the Related Art
In an ink jet recording apparatus (hereinafter also referred to simply as a “recording apparatus”), a desired recording operation is performed by ejecting ink droplets to fly out from fine ejection orifices, which are formed in an ink jet recording head (hereinafter also referred to simply as a “recording head”), such that the ink droplets impact a recording medium. In an recording apparatus for recording prints which are put up outdoors, a pigment ink is used for the recording because prints recorded using a dye ink have poor resistance to light and weather.
Pigment ink is prepared by dispersing pigment particles in a solvent. Therefore, when the ink is held in the same posture and state for a long time, the pigment particles tend to precipitate under the influence of gravity. Precipitation of the pigment particles in the pigment ink causes a density gradient in the direction of gravity such that the density of the pigment particles is relatively low in an upper part of the ink and relatively high in a lower part of the ink, as viewed in the direction of gravity. Because shades of the pigment ink depend on the density of the pigment particles, a part of the ink containing the pigment particles at a higher density provides a relatively dark (deep) color and a part of the ink containing the pigment particles at a lower density provides a relatively light (pale) color. Accordingly, a visually recognizable difference in shades occurs in comparison between a print recorded by using the pigment ink in the state where the pigment particles are precipitated and a print recorded by using the pigment ink in the state where the pigment particles are not precipitated and the pigment density is uniform. When a print is recorded using the pigment ink in the state where the pigment particles are precipitated, there is a possibility that color balance differs between a beginning portion and an ending portion in one page of print. To overcome the above-mentioned problems, a proposal for avoiding the precipitation of pigment particles has been made in which an ink is stirred by providing an ink stirring member within an ink tank.
FIG. 7 illustrates an on-carriage ink tank disclosed in Japanese Patent Laid-Open No. 2008-273043. FIG. 7 is a vertical sectional view of the disclosed on-carriage ink tank in a posture that an ink reservoir is mounted to an ink jet recording apparatus. Two swingable members 100 and 110 are disposed within the ink reservoir. Ink flows are generated, as indicated by arrows TA, inside the ink tank with those swingable members 100 and 110 swinging in response to movements of a carriage onto which the ink tank is mounted. The swingable member 100 generates a rising ink flow, and the swingable member 110 generates an ink flow advancing toward the swingable member 100 along a bottom surface of the ink reservoir. With such an arrangement, the ink residing near the bottom surface of the ink reservoir and containing the pigment component at a higher density is raised upwards from the bottom surface. As a result, the ink within the ink reservoir can be efficiently stirred.
In a large-sized ink jet recording apparatus recently commercialized, an off-carriage ink tank having a large capacity of an ink reservoir is used to reduce the replacement frequency of the ink tank. To allow replacement of the ink tank even during recording, this type of ink jet recording apparatus is constructed such that a sub-tank capable of temporarily reserving ink is disposed inside the recording apparatus and the ink is supplied from the sub-tank to a recording head through an ink supply passage formed of a tube. In the ink jet recording apparatus thus constructed, pigment particles precipitate in the pigment ink reserved in the ink supply passage and the sub-tank as well. To cope with the precipitation of the pigment particles, the ink in the ink supply passage and the sub-tank is circulated to stir the ink, to thereby prevent the pigment particles from precipitating in the ink.
FIG. 8 illustrates an ink jet recording apparatus disclosed in Japanese Patent Laid-Open No. 2008-55646. The ink jet recording apparatus illustrated in FIG. 8 includes a sub-tank 2, and a first ink supply passage 8 for circulating ink while bypassing an ink jet head. In the ink jet recording apparatus disclosed in Japanese Patent Laid-Open No. 2008-55646, a circulation pump 9 disposed in the first ink supply passage 8 is operated to suck up the ink within the sub-tank 2 and to convey the ink along circulation routes in sequence, as indicated by arrows a, b, c and d in FIG. 8. Thus, in the ink jet recording apparatus disclosed in Japanese Patent Laid-Open No. 2008-55646, the ink in the first ink supply passage 8 and the ink in the sub-tank 2 can be stirred by returning a large volume of the ink to the sub-tank 2 at a high speed. As a result, pigment particles can be prevented from precipitating in the ink. Further, a magnet stirrer 15 is disposed within the sub-tank 2 to stir the ink in the sub-tank 2, thereby further preventing precipitation of the pigment particles in the ink.
However, the construction disclosed in Japanese Patent Laid-Open No. 2008-273043 is limited in its application to the ink jet recording apparatus employing the on-carriage ink tank, and it cannot be applied to the case of stirring the ink reserved in the sub-tank which is fixedly mounted to a main body of the recording apparatus.
Also, the construction disclosed in Japanese Patent Laid-Open No. 2008-55646 requires a passage for stirring the ink to be provided separately from an ink supply path for the recording operation. In other words, the passage arrangement is complicated, which increases the size and cost of the apparatus.